In ink jet printing systems the print is made up of individual droplets of ink generated at a nozzle and propelled towards a substrate. There are two principal systems: drop on demand where ink droplets for printing are generated as and when required; and continuous ink jet printing in which droplets are continuously produced and only selected ones are directed towards the substrate, the others being recirculated to an ink supply.
Continuous ink jet printers supply pressurised ink to a print head drop generator where a continuous stream of ink emanating from a nozzle is broken up into individual regular drops by, for example, an oscillating piezoelectric element. The drops are directed past a charge electrode where they are selectively and separately given a predetermined charge before passing through a transverse electric field provided across a pair of deflection plates. Each charged drop is deflected by the field by an amount that is dependent on its charge magnitude before impinging on the substrate whereas the uncharged drops proceed without deflection and are collected at a gutter from where they are recirculated to the ink supply for reuse. The charged drops bypass the gutter and hit the substrate at a position determined by the charge on the drop and the position of the substrate relative to the print head. Typically the substrate is moved relative to the print head in one direction and the drops are deflected in a direction generally perpendicular thereto, although the deflection plates may be oriented at an inclination to the perpendicular to compensate for the speed of the substrate (the movement of the substrate relative to the print head between drops arriving means that a line of drops would otherwise not quite extend perpendicularly to the direction of movement of the substrate).
In continuous ink jet printing a character is printed from a matrix comprising a regular array of potential drop positions. Each matrix comprises a plurality of columns (strokes), each being defined by a line comprising a plurality of potential drop positions (e.g. seven) determined by the charge applied to the drops. Thus each usable drop is charged according to its intended position in the stroke. If a particular drop is not to be used then the drop is not charged and it is captured at the gutter for recirculation. This cycle repeats for all strokes in a matrix and then starts again for the next character matrix.
Ink is delivered under pressure to the print head by an ink supply system that is generally housed within a sealed compartment of a cabinet that includes a separate compartment for control circuitry and a user interface panel. The ink may be mixed with a solvent, for example to assist in the control of the viscosity of the ink-solvent mixture.
As mentioned above, a continuous stream of ink is broken up into individual regular drops by, for example, an oscillating piezoelectric element. The number of drops generated per second is proportional to the oscillation frequency of the piezoelectric element. The piezoelectric element is typically driven at or near to its resonant frequency. The resonant frequency is controlled (in other words, tuned) to ensure that it is equal to or near a predetermined driving frequency, the predetermined driving frequency being chosen to ensure that a specific number of drops are generated per second. The mass of the piezoelectric element may be increased or decreased to alter its resonant frequency.
Controlling the resonant frequency of the piezoelectric element by changing its mass is a skilled and time consuming task, usually undertaken by skilled technicians. It is therefore usual for the entire print head to be replaced with a new print head having a correctly tuned piezoelectric element, or for the entire print head to be sent away (e.g. to the manufacturer of the print head or piezoelectric element) to have a newly tuned piezoelectric element installed. This is costly, and may also result in the printer being inoperable for a period of time. The replacement and/or reinstallation may need to be undertaken periodically, for example to take into account changes in environmental conditions, due to, for example, relocation of the printer or print head.
The distance from the nozzle at which the continuous stream of ink breaks up into individual regular drops (i.e. the break up point) is dependent upon many factors. One factor which has an effect on the location of the break up point is the magnitude of the oscillations of the oscillating piezoelectric element. The magnitude of the oscillations of the piezoelectric element are proportional to the magnitude of the modulating voltage which drives the oscillating piezoelectric element. By increasing or decreasing the magnitude of the modulation voltage, the break up point can be moved relative to the nozzle from which the continuous stream of ink emanates. However, the relationship between modulation voltage and the distance from the nozzle at which break up occurs (often referred to as the break up length) is not always a directly proportional relationship.
In many cases, an increase in the magnitude of the modulation voltage will result in a decrease in the break up length up to a certain point, after which further increases in the modulation voltage will result in a decrease of the break up length. The point at which the break up length stops decreasing (or increasing) and begins to increase (or decrease) is often referred to as a turning point. Selection of the magnitude of the modulation voltage to ensure that break up of the continuous stream into individual droplets occurs around this turning point is advantageous. In the region around the turning point, the formation of satellite drops is reduced or eliminated. Satellite drops are much smaller and often more irregularly shaped drops which accompany the regular drops breaking out of the continuous stream. Such satellite drops can lead to a reduction in print quality, and it is therefore desirable to reduce or eliminate them. It is often preferred to choose a modulation voltage which does not result in a break up length which coincides with the turning point. This is because a break up length which coincides with the turning point may be unstable. In previous continuous ink jet printers, it is therefore known to first identify a turning point, and to then choose a modulation voltage which results in a break up length which is slightly offset from the turning point.
The exact position of the turning point is dependent on a number of factors, for example the ink and solvent used, the temperature of the ink-solvent mix, and the viscosity of the ink-solvent mix. In some cases, a turning point may not be detected in the operating modulation voltage range of the oscillating piezoelectric element. Even for ink-solvent mixtures which do normally exhibit a turning point in the range of operating modulating voltages, the turning point may not be detected due to changes in conditions of, for example, the ink-solvent mixture. If a turning point cannot be identified, the known method of identifying a turning point and choosing a modulation voltage which results in a break up length slightly offset from the turning point is not workable.
In the prior art, a turning point is identified, and then a modulation voltage is chosen which results in a break up length slightly offset from the turning point. This chosen modulation voltage is then applied to the oscillating piezoelectric element. This modulation voltage will be applied to the oscillating piezoelectric element continuously while the machine is running. In other words, the modulation voltage will not be changed. If the break up point of the continuous stream of ink moves (or, more generally, the break up point-modulation voltage characteristic changes) the applied modulation voltage may no longer result in an acceptable print quality. For example, if the break up point-modulation voltage characteristic changes, for example, due to changes in temperature, the previously calculated modulation voltage may coincide with a point on the characteristic which is no longer sufficiently near a turning point to achieve little or no satellite drop generation. The characteristic may change so much that, at the applied modulation voltage, the break up point of the continuous stream of ink is no longer within or in the vicinity of the charge electrode. This may mean that drops emerging from the continuous stream of ink may not be charged as required, or charged at all, again having a detrimental effect on print quality.